Current diversification of functions of electronic devices have put electronics using semiconductors into practical use in various fields including household devices, data processing devices, communication systems and industrial devices. Semiconductors such as ICs, LSIs, thyristors and so forth used in these electronics have excellent performance but are very sensitive to abnormal voltage which may be applied by electrostatic discharges, lightning and so forth, so that troubles such as malfunction or breakdown of the electronic devices tend to be caused when abnormal voltage is exerted thereto. It is therefore a matter of great significance to take a suitable measure against surge voltages from the view point of security and an improvement of the electronic devices.
FIGS. 6(a), 6(b) and 6(c) show an example of a known surge absorber. Referring to these Figures, a reference numeral 1 denotes a tabular varistor element usually made of a semiconductor ceramics mainly composed of zinc oxide, strontium titanate or the like, while 2 denotes electrodes which are provided on the corresponding portions of the front and rear sides of the varistor element 1 by, for example, firing of silver paste. The electrode 2 on the rear side of the varistor element 1 is not shown in these Figures. Numerals 3a and 3b show metallic electrode plates made of materials which have a high degree of electrical conductivity such as copper or brass. The electrode plates 3a and 3b are connected to the electrodes 2 by soldering of solder paste. A reference numeral 4a designates an external terminal which is led from a portion of the electrode plate 3a. Usually, the external terminal 4a is made of the same material as the electrode plate 3a. This applies also to another external terminal 4b provided on the rear side. The ends of the external terminals 4a and 4b are connected to an electric circuit by soldering or by means of bolts. Practically, the surge absorber is resin-coated or resin-molded such that only the ends of the external terminals are exposed, though not shown in FIGS. 6(a) to 6(c).
The operation of this known surge absorber will be described hereinunder.
The external terminals 4a and 4b are connected between the power supply lines, signal lines or grounding lines of the device to be protected, so as to absorb any abnormal voltage which has been introduced into these lines due to, for example, electrostatic discharge, lightning surge, and so forth. The surge current produced by the abnormal voltage flows from the external terminal 4a on the front side of the surge absorber to the external terminal 4b on the rear side thereof, through electrode plate 3a, electrode 2, varistor element 1, and the corresponding portions on the rear side of the surge absorber so that the voltage suppressed to a safe level is applied to the device to be protected.
This known arrangement, however, suffers from a disadvantage in connection with the production. Namely, in producing the surge absorber, external terminals 4a and 4b are held by means of jigs so as to support the varistor element 1 therebetween by spring action of the external terminals 4a, 4b during soldering of the electrodes 2, 2 and the electrode plates 3a, 3b together. In such a case, the varistor element 1 is supported solely by the external terminals 4a and 4b only at the free ends thereof so that only a slight variation in the thickness of the varistor element 1 causes uniform contact between the electrode plates 3a, 3b and the electrodes 2 on the element 1 to be not ensured, resulting in that the soldering cannot be done uniformly.
Even if the electrode plates 3a, 3b are held in uniform contact with the electrodes 2 on both sides of the element 1, flux voids tend to remain between the electrode plates 3a, 3b and the electrodes 2, thus making it difficult to ensure uniformity of the soldering.
These unfavorable factors undesirably impair performance in regard to withstanding surge current capacity, which is one of the most critical requirements for a surge absorber, resulting in a lowered reliability of the surge absorber.
The soldering of the electrode plates 3a, 3b to the electrodes 2 is usually conducted by printing paste solder on the surfaces of the electrodes 2, drying the paste solder and pressing the electrodes plates 3a, 3b onto the electrodes 2 under application of heat. Such a method, however, is very expensive.